Rare? Most of us are eventually going to die of illnesses with fairly miserable
end states (some heart disease, most cancers, stroke, Alzheimer's). I'd be quite
surprised if the expected utility for the last couple of months of a typical life isn't negative.
Do you really think that this situation is rarer than, say, 25%? For the 25% of us with
the worst last two months in the populations as a whole, do you think that the utility of
those two months is actually positive?

(I'm picking two months as the time interval from watching two relative's deaths from
Alzheimer's and pancreatic cancer respectively. It seems like a conservative estimate
of the amount of time spent with net negative quality of life in their cases. It seems like
a reasonable guess at a conservative estimate for typical terminal illnesses.)

I'm considering the possibility of an experimental treatment becoming available during those two months that could save the terminally ill patient from dying of that illness. Being alive would then allow the possibility of new life extension treatments, would could lead to a very long life indeed.

This would be a conjunction of possibilities, so I realize that the overall possibility of a terminally person transitioning to a very long-lived person is slim, but even a slim chance of living for a very long time is worth almost any degree of suffering. If no experimental treatment becomes available during those two months (the likely outcome), cryonics upon death is the next best legal option. If suicide + cryonics were legal, it would make sense to try that if no experimental treatment were even in the research pipeline, but it's not legal, and so no cryonics organization would go through with it.

Also, as a transhumanist, I don't accept that most of us are eventually going to die of illnesses with fairly miserable end states.

It seems extremely unlikely that an experimental treatment will appear as a surprise within two months. If it's actually new, then there will be trials of it first, and I think research could turn up that information.

but even a slim chance of living for a very long time is worth almost any degree of suffering

Huh? Are you applying any discount rate to the value of living a very long time?
The tradeoffs you are describing sound like they are calculated with the current utility
of a very long lifespan being almost unbounded. For someone with a discount rate of 1% annually,
an infinite lifespan has a net present utility of 100 years of lifespan. If, for instance, there was a 0.1%
chance of the conjunction of a cure and an indefinite lifespan it wouldn't be worth -0.11 lifespan-years of utility,
and a miserable two months could easily match that.

Also, as a transhumanist, I don't accept that most of us are eventually going to die of illnesses with fairly miserable end states.

It isn't desirable, of course. Nonetheless, looking, for instance, at the rather modest progress since the "war on cancer" was announced 40 years ago, it seems like a plausible extrapolation. Of course, a uFAI is perhaps plausible, and would technically satisfy your claim, a paperclipped population doesn't get cancer, but I don't think that is what you intended... What do you intend, and what is your evidence?